Medical practitioners have acknowledged a great need for implantable cardiac assist devices for supplementing the blood pumping action of a diseased or failing heart. It has been estimated that over 400,000 people in the United States could benefit from permanently implantable cardiac assist devices. Such devices not only work to alleviate bodily dysfunctions attributable to reduced blood pumping ability of the failing heart, but they can also promote the healing of the diseased or failing heart by reducing its required contractile load and can protect other organs from failure due to poor circulation.
One cardiac assist device known in the art is the left ventricular assist pump. That device consists of a pump with a pumping chamber connected via a blood inlet to the left ventricle of a failing heart and via a blood outlet to the aorta. In operation, the pumping chamber fills with blood in response to blood pressure from a natural contraction of the left ventricle. After the pump chamber is filled and the heart is in diastole, the blood inlet is closed by a unidirectional pressure controlled valve that blocks the flow of blood from the pumping chamber back through the inlet into the left ventricle of the failing heart. Subsequent to closure of the valve, the pumping chamber is compressed by the action of a power source. The blood contained in the pumping chamber is ejected from the pumping chamber through the outlet valve into the aorta, thereby supplementing the pumping action of the failing heart. The pumping action is coordinated with natural heart function with the aid of sensors positioned to monitor electrical activity of the heart.
The widespread use of the ventricular assist pump or similar types of cardiac assist devices has been greatly limited by the lack of suitable power sources. Under normal physiologic conditions for adults at rest, about 3 to about 5 watts of power are required to pump blood. Even though cardiac assist devices only supplement and do not completely replace the pumping action of the heart, it is important that they reliably deliver substantial power for periods of years without recharging, replacement or maintenance. One proposed power source that has many of these features is skeletal muscle. The contractile force produced by electrically stimulated autogenous skeletal muscle can be used to power an implanted blood pump.
As disclosed in U.S. Pat. No. 4,813,952, issued Mar. 21, 1989, to Khalafalla, expressly incorporated herein by reference, the use of autogenous skeletal muscle to power a cardiac assist blood pump is known in the art. Commonly employed skeletal muscles include the rectus abdominis, the latissimus dorsi, or other muscles that can be translocated from their normal positions with their associated neurovascular supply left intact, and wrapped around a blood reservoir, typically a flexible pouch, having blood conduits connecting it to the circulatory system. Blood pumping action of such cardiac assist devices is powered by the contractile forces exerted bY the innervated skeletal muscle in response to electrical stimulation of the appropriate skeletal muscle nerves. In operation, the flexible pouch fills with blood from the circulatory system, stretching the skeletal muscles wrapped around the pouch. Timed electrical pulses produced by a pulse generator trigger the contraction of the skeletal muscle. The timing of the electrical pulses is typically coordinated with sensed natural heart activity. As the flexible pouch is compressed by the contracting skeletal muscle, the blood contained in the pouch is pumped into the aorta or other major artery of the patient's circulatory system, thereby supplementing the natural blood pumping action of the heart.
The overall function and efficiency of such cardiac assist devices depends on a number of factors including the configuration of the flexible pouch or blood reservoir and its connection to the circulatory system, the identity and condition of the muscle used to wrap the flexible pouch, amplitude, frequency and duration of the electrical pulses used to trigger muscle contraction, and coordination of pumping action with the natural pumping action of the heart. With optimization of such variables cardiac assist devices have been able to deliver as much as 20% to 80% of the pumping activity of a normal heart, but only for brief periods of time. Skeletal muscle fatigues rapidly when it is required to contract rhythymically at rates matching the normal heart contraction rate. Because the volume of blood pumped by this type of cardiac assist device rapidly diminishes as the skeletal muscle becomes fatigued, clinical use of skeletal muscle powered cardiac assist devices has been limited.
It is therefore an object of this invention to provide a method for minimizing fatigue in skeletal muscles used to power cardiac assist pumps.
It is another object of this invention to provide an apparatus for minimizing fatigue in skeletal muscle in use for powering a cardiac assist device having a blood reservoir and a skeletal muscle positioned to compress the blood reservoir in response to a muscle contracting stimulus.
Still another object of this invention is to provide a method and apparatus for increasing blood circulation in skeletal muscle in use for powering cardiac assist devices.
In accordance with the foregoing objectives, there is provided an apparatus for enhancing circulation of blood through skeletal muscle in use for powering a cardiac assist device designed to provide blood pumping action to supplement the pumping action of a diseased or failing heart. The device includes a blood reservoir in fluid communication with a patient's circulatory system. The skeletal muscle used for powering the cardiac assist device is positioned to compress the blood reservoir in response to a muscle contracting stimulus typically produced by a pulse generator programmed to initiate the signal in a predetermined time relationship to sensed heart function. Blood circulation within the skeletal muscle is enhanced by use of a valve positioned to control blood flow from the patient's circulatory system into the blood reservoir. The valve is opened and closed in a timed relationship to the skeletal muscle contracting stimulus and/or heart activity. More particularly, the valve is controlled to close immediately before and for a predetermined, but preferably programmable, time period following contraction of the skeletal muscle.
Without the use of the valve to block flow of blood from the patient's circulatory system into the blood reservoir for a period of time following each muscle contraction, blood would immediately begin filling the blood reservoir following muscle contraction. The pressure exerted by the blood reservoir against the skeletal muscle as the reservoir fills with blood is sufficient to compress the arterioles and other small blood vessels distributed through the skeletal muscle, thereby diminishing blood flow and the transport of oxygen and other needed nutriments to, and transport of waste products from, skeletal muscle cells. This diminished transport results in muscle fatigue. Delaying the flow of blood into the blood reservoir for a predetermined length of time after each muscle contraction lengthens the time between muscle contractions during which blood can circulate in the skeletal muscle. This enhanced blood circulation has been found to greatly reduce fatigue of the muscle. More time is allowed for delivery of necessary nutriments to skeletal muscle cells, and removal of waste products. The consequent reduction in skeletal muscle fatigue enables continuous use of skeletal muscle powered cardiac assist devices at contraction rates approaching the heart contraction rate with a long term, sustained blood pumping capacity.
In a preferred embodiment, skeletal muscle such as a rectus abdominis along with its associated neurovascular system is translocated from its normal position in a patient's body and wrapped around a blood reservoir, typically a flexible pouch. The flexible pouch is in fluid communication with the patient's circulatory system through a blood outlet conduit and a blood inlet conduit. The blood outlet conduit includes a unidirectional valve placed to prevent blood flow from the patient's circulatory system into the blood reservoir. A valve is positioned to control blood flow into the blood reservoir from the patient's circulatory system through the blood inlet conduit. In a most preferred embodiment, the valve is biased to remain in a closed position until activated. The valve can be activated to the opened position by a signal from a pulse generator. The valve is opened and closed during operation of the cardiac assist device in a timed relationship to the muscle contracting stimulus.
Thus, prior to each skeletal muscle contraction, and optionally in coordination with heart activity, the valve is opened for a controlled period of time (the valve open time) to allow blood to flow from a patient's circulatory system into the flexible pouch. Blood flowing into the flexible pouch causes it to expand, thereby applying pressure to and stretching the skeletal muscle wrapped around the pouch. Controlling the amount of blood flowing from the patient's circulatory system into the flexible pouch (by controlling the valve open time) allows selection of the desired precontraction pressure exerted by the flexible pouch against the skeletal muscle. Increasing the precontraction pressure on the skeletal muscle, up to a limit, acts to increase the available contractile force that can be exerted by the skeletal muscle. The valve is closed to block blood flow in the blood inlet conduit prior to application of a muscle contracting stimulus to the skeletal muscle. Contraction of the skeletal muscle in response to the muscle contracting stimulus forces the blood contained in the flexible pouch through the blood outlet conduit into the patient's circulatory system. Repetition of this sequence supplements the blood pumping action of the circulatory system of a patient's body. Optionally, the muscle contracting stimulus can be applied to the skeletal muscle in coordination with the pumping action of the heart, as sensed by a heart sensor suitably positioned to monitor heart activity.
Fatigue of skeletal muscle in use for powering a cardiac assist device having a valve to control the flow of blood into the blood reservoir can also be reduced by applying to the muscle a training regimen in the form of an electrical stimulus to promote conversion of fast twitch muscle fibers into slow twitch muscle fibers. Slow twitch muscle fiber has a greater tolerance for long term contractile loading than does fast twitch muscle fiber. Since about 50% of a typical skeletal muscle is composed of the easily fatigued fast twitch muscle fiber, skeletal muscle fatigue in cardiac assist devices can be reduced with conversion of fast twitch muscle fibers into slow twitch muscle fibers. That conversion can be promoted by a muscle training regimen that includes applying to the skeletal muscle an electrical stimulus having a predetermined amplitude, frequency, and duration. The preferred training regimen includes the application of series of electrical pulses to cause muscle twitching, alternated with a train of optionally ramped electrical pulses to cause a smooth tetanic muscle contraction. Conversion of fast twitch muscle fibers into less easily fatigued slow twitch muscle fibers is essentially complete after a six week training regimen. The training regimen permits cardiac assist function during the training period. Such muscle training can therefore be done "on the job", after implantation of the cardiac assist device and during its operation.
The present invention enables implantation of skeletal muscle powered cardiac assist devices for long term operation. Previous devices of that type were limited because of power considerations and muscle fatigue. The present method and apparatus allows enhanced perfusion of the skeletal muscle to reduce muscle fatigue by delaying the flow of blood into a blood reservoir for a predetermined time following each muscle contracting stimulus applied to skeletal muscle positioned to compress the blood reservoir.
The present invention is readily adapted to art recognized cardiac assist devices that use skeletal muscle power. Valves that can be controlled to block or permit flow of blood through a blood inlet into a blood reservoir are commercially available. Such a valve can be used for example, to replace the unidirectional valve in the blood inlet conduit of the cardiac assist device described in U.S. Pat. No. 4,813,952. Further, programmable heart sensor/pulse generators are available and readily adapted for use in accordance with this invention.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art on consideration of the following detailed description of preferred embodiments.